Diluted aqueous solutions of hydrogen fluoride (HF) are generated during many chemical processes; significant streams are generated by electrochemical fluorination operations or by thermal oxidizers incinerating fluorine-containing off-gas streams. The off-gases are usually scrubbed and the resulting aqueous streams have a concentration of hydrogen fluoride of 0.5 to 40%. These HF solutions cannot be released into the environment without pre-treatment, nor does a waste water treatment tolerate such solutions.
Consequently, these HF streams are treated with aqueous solutions of calcium hydroxide (Ca(OH)2). In this process, calcium fluoride (CaF2) is precipitating. However, the precipitated CaF2 particles are very small, the particle size is usually below 20 μm. To improve the handling properties of the calcium fluoride, organic precipitation agents (flocculants) are added. The final sludge is filtered and land-filled or supplied to cement or steel industry as processing ingredient. Due to the fine particles sizes and the organic residues such calcium fluoride cannot be used to manufacture anhydrous hydrogen fluoride (AHF).
Attempts to generate larger particles of calcium fluoride during the treatment of Ca(OH)2 all failed due to the low solubility product of CaF2.
EP 0 536 051 A1 discloses a method for recovering calcium fluoride from a fluoroetchant. Calcium fluoride of high purity is produced from etchants, mainly composed of hydrogen fluoride, silicon compounds and ammonium fluoride. Calcium fluoride is produced by a reaction of hydrogen fluoride with calcium carbonate at a temperature not lower than 50° C. The reaction is carried out as a two-step process; in the first step a granular calcium carbonate with a grain size of 0.05 to 0.15 mm is used, in the second step a calcium carbonate powder with a grain size of 3 to 30 μm is used. Due to the calcium carbonate powder used for this process, the resulting calcium fluoride contains a significant amount of fine calcium fluoride particles with a grain size of 3 to 30 μm. Due to the fine calcium fluoride particles, it is difficult to use the product in existing manufacturing processes for anhydrous hydrogen fluoride, as the fine particles cannot be processed with sulfuric acid as they would generate a cloud of dust which cannot be tolerated in the manufacturing process of anhydrous hydrogen fluoride. Another disadvantage of this process is the required temperature of ≧50° C. The energy cost for heating the diluted HF makes the process non-economical.
A need still exists for a process for producing free-flowing calcium fluoride from diluted aqueous solutions of hydrogen fluoride which can be used in existing manufacturing processes for anhydrous hydrogen fluoride.